D1: Principles of Star Creation
A nebula is an enormous cloud of dust and gas occupying
the space between stars and acting a nursery for new stars.
Nebulae are made up of dust, basic elements such as
hydrogen and other ionised gases. Clouds of gas are
common in our galaxy and other galaxies like ours. These
clouds are called nebulae, a typical nebula is many light
years across and contains enough mass to make several
thousand stars the size of the sun.
Gradually the mutual gravitational attraction between all the particles and molecules in the
nebula causes the cloud to collapse inwards. This process is called accretion. The
temperature of the nebula increases as it collapses. As the cloud shrink, its gravitational
potential energy is converted to the kinetic energy of the individual gas particles falling
inwards. The flattening is a natural consequence of collisions between particles in a spinning
cloud.
In Astrophysics, the Jean’s instability
causes the collapse of interstellar gas
clouds and subsequent star
formation, named after James Jeans,
a British astrophysicist. It occurs
when internal gas pressure is not strong enough to prevent gravitational collapse of a region
filled with matter. If the mass of the nebula is greater than Jeans’ Mass than a nebula will
collapse.
Fragmentation
The collapsing cloud will break into tens, hundreds or thousands of fragments. Each fragment
continues to collapse under its own gravity. The temperature within each fragment rises
further as the gravitational energy is converted to kinetic energy of particles.
Protostar
A very young star that is still gathering mass from its parents
molecular cloud. The protostar phase is the earliest one in the
process of stellar evolution. For the low-mass stars, it will last
around 500,000 years. Within its deep interior, the protostar has
lower temperature than an ordinary star as at its centre
hydrogen is not yet fusing with itself. Theory predicts that the
hydrogen isotope, however, fuses with Hydrogen 1 creating, helium 3.
Equilibrium in a Protostar
1. Gravity pulls gas and dust inwards towards the core
2. Inside the core, temperature increases as gas atoms collisions increase.
, 3. Density of core increases as more atoms share space
4. Gas pressure increases as atomic collisions and density increase
5. The protostar’s gas pressure resists the collapse of the nebula
6. When gas pressure = gravity, protostar has reached equilibrium and accretion stops.
For a protostar, equilibrium occurs when gas pressure equals gravity. Gravity remains
constant so gas pressure relies on two things to maintain it: a very hot temperature and
density.
Initial Nuclear Reactions
Nuclear fusions begins to occur in a dense centre of collapsed
nebula when temperature exceeds 10^6 Kelvin. Small nuclei
collide with each other with sufficient energy to overcome
the electrostatic repulsion between their positive nuclear
charges. These nuclei combine to form a larger nuclei.
Deuterium is the most easily fused nucleus. Hydrogen and
Deuterium produces Helium 3 nucleus as photons carry light
energy away from a reaction.
Timings
Stage number Stage Name Time
1 Accretion of Nebula 2x10^6 years
2 Fragmentation 3x10^4 years
3-4 Protostar formation 1x10^5 years
5 Main Sequence Evolution 1x10^7 years
Main Sequence Stars
Protostars eventually evolve into main sequence stars.
Main sequence stars fuse hydrogen atoms to form
helium in their cores. During main sequence there is
an equilibrium of forces inside the star, which
maintain its size and temperature for a long period of
time. About 90 percent of the stars in the universe are
main sequence. These stars range from a tenth of the
mass of the sun to 200 times as big. The size of the star
depends on how big the original nebula was. Main
Sequence stars are in hydrostatic equilibrium. That is
the inward force of gravity which tends to compress
the star which is balanced by the outward force due to
pressure. The star is sustained by steady hydrogen
fusion reactions in the core.
D2: Principles of Star Death
Life cycle of a star
• Begins as forming nebula
• Then it becomes protostar
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